Crystal structure of the ternary silicide Gd2Re3Si5

The crystal structure of this ternary silicide belongs to the U2Mn3Si5 structure type. The coordination polyhedra of the Gd atoms have 21 vertices, while those of the Re atoms are cubooctahedra and 13-vertex polyhedra, and the Si atoms are arranged as tricapped trigonal prisms, bicapped square antiprisms, or 11-vertex polyhedra.

A single crystal of the title compound, the ternary silicide digadolinium trirhenium pentasilicide, Gd 2 Re 3 Si 5 , was isolated from an alloy of nominal composition Gd 20 Re 30 Si 50 synthesized by arc melting and investigated by X-ray single-crystal diffraction. Its crystal structure belongs to the U 2 Mn 3 Si 5 structure type. All atoms in the asymmetric lie on special positions. The Gd site has site symmetry m..; the two Mn atoms have site symmetries m.. and 2.22; the three Si atoms have site symmetries m.., ..2 and 4.. . The coordination polyhedra of the Gd atoms have 21 vertices, while those of the Re atoms are cubooctahedra and 13-vertex polyhedra. The Si atoms are arranged as tricapped trigonal prisms, bicapped square antiprisms, or 11-vertex polyhedra. The crystal structure of the title compound is also related to the structure types CaBe 2 Ge 2 and W 5 Si 3 . It can be represented as a stacking of Gd-centred polyhedra of composition [GdSi 9 ]. The Re atoms form infinite chains with an Re-Re distance of 2.78163 (5) Å and isolated squares with an Re-Re distance of 2.9683 (6) Å .

Structural commentary
The existence of the compound Gd 2 Re 3 Si 5 has been reported earlier (Bodak et al., 1978). The unit-cell parameters were determined and the structure type was assigned. A complete investigation of the crystal structure by X-ray single crystal diffraction has now been undertaken. The coordination polyhedra of the Gd atoms have 21 vertexes, whereas those of the Re atoms are cubooctahedra or 13-vertex polyhedra, and the Si atoms tricapped trigonal prisms, bicapped square anti-ISSN 1600-5368 prisms, or 11-vertex polyhedra. The U 2 Mn 3 Si 5 -type structure is closely related to the structure type BaAl 4 and its ordered derivative CaBe 2 Ge 2 . In particular, the U 2 Mn 3 Si 5 -type can be considered to be formed by one-dimensional structural fragments of the structure type CaBe 2 Ge 2 , running parallel to the direction [00l]. There also exists a relationship between the structure types U 2 Mn 3 Si 5 and W 5 Si 3 . Fragments which can be viewed as deformed square antiprisms are common to both structures. The crystal structure of Gd 2 Re 3 Si 5 can also be represented as a stacking of Gd-centred polyhedra of composition [GdSi 9 ], located at z = 0 and 1 2 ( Fig. 1) (Parthé et al., 1993). The Re atoms form infinite chains with an Re-Re distance of 2.78163 (5) Å and isolated squares with an Re-Re distance of 2.9683 (6) Å .

Synthesis and crystallization
An alloy of nominal atom percent composition Gd 20 Re 30 Si 50 was synthesized from the high-purity elements by arc melting on a water-cooled copper plate under a purified argon atmosphere, using titanium as a getter and a tungsten electrode. The weight loss during the sample preparation was less than 0.5% of the total mass (1 g). The alloy was placed into an Al 2 O 3 crucible and inserted into a tantalum container, which was then sealed by welding, leaving the sample under an argon atmosphere. The sample, wrapped in tantalum foil, was heated to 1623 K in a muffle furnace at a rate of 200 K h À1 , held at this temperature for 5 h and then cooled to room temperature at a rate of 50 K h À1 .

Refinement details
A single crystal of well-defined shape was separated from the sample. The structure was solved by direct methods. The highest Fourier difference peak of 2.35 e Å À3 is at (0, 1 2 , 1 4 ), 0.00 Å away from atom Re2. The deepest hole (À2.44 e Å À3 ) is at (0.6045, 0.3985, 0), 1.52 Å away from the Gd atom. Details of the crystal parameters, data collection and the structure refinement details are summarized in Table 1. Stacking of Gd-centred polyhedra in the structure of the compound Gd 2 Re 3 Si 5 with displacement ellipsoids drawn at the 99% probability level.  (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 2012); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Special details
Experimental. Analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by Clark & Reid (1995). Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Gd 0.26249 (5)